I’m having a bit of a weird issue with my KT100 engine. This issue first started happening when I was practicing 2 weeks ago. In 2 of the left turns my engine would stumble or miss slightly. Wasn’t really sure what was causing it. I thought it may have been low fuel at first, but that wasn’t the problem. I had the tank about 3/4 of the way full which should be more than enough. Unfortunately while I was trying more fuel in the kart my seat strut snapped and that ended my chances to try anything else.

I thought it might have been a bad spark plug as my engine was also down about 500 RPM during those practice runs. So I figured a new plug would fix the RPM problem and the left turn stumbling problem. We had a race yesterday so I put a brand new plug in, it fixed the RPM issue but not the left turn stumbling/missing problem.

The weird thing is that this issue only starts happening after I’ve ran about 4-5 mins. So I’m assuming it is not something that is simply loose. I’m also assuming its not a problem with something in the fuel line or pulse line as I would think that would cause the issue to happen as soon as I get on the track. Its almost like it is something that is triggered by temperature. It got progressively worse during the final yesterday.

The only change I made before my practice session 2 weeks ago was to increase the clutch stall speed by about 200-300 rpm so now its at 8500. Not sure if I somehow messed up something there or not. I wonder if 8500 is just a bad spot for it?

Here is a video of the entire final. The issue starts happening around the 5:30 min mark and after 12 mins the stumbling/missing is very noticeable in left turns.

Tim, its funny you mentioned that because I had the same exact issue in our 2nd race of the season lol. That was just a case of me being a noob and not checking my bearing set screws. Had a few of them fall out and the axle shifted and rotor started rubbing the brake pads. That was more of a gradual feeling though and it bogged the engine down. This new issue with left turns its like the engine completely shuts off for a split second.

Thanks Gary. You might be right about the airbox. I’ve heard that issue mentioned before and it never even crossed my mind that it could be what my problem is too. Maybe I lay my leg down a bit more after 5 laps. I do know that the airbox holes are pointed towards my leg a bit. Hope that is the problem, simple fix 🙂

I’ll check on that pickup tube too. It’s been on the kart since I bought it in October.

If its not the airbox, observe the rev counter (or review the data) and see if it behaves oddly when this happens. It does sound like an ignition issue. They tend to crop up when the motor gets some heat into it and disappear when its cold.

That’s a carburetion problem. You don’t see it too often but it happens in a certain series of turns/straights with some set ups. The track empties out the crank case and you open the throttle too fast and no fuel. Usually a long straight tight turn short straight another turn and when you get on it nothing.

I’ve been working through something just like this. So far I’ve tried gearing shorter and playing games with the pedal in the problem corner, both of which do help. I’ve checked the frame for twist and it’s slightly twisted the wrong way to cause my issue, it’s lifting the inside rear more in that direction, maybe it drops too soon. Obsessively clean the power valve.

Anyway my scenario matches Walt’s description, a fast S, short straight, then hairpin. What would you do to the carb? My guess is:
Clean passages.
Raise the fuel level in the bowl.
Replace needle valve.

Mike, another EKN member messaged me today and said that the low speed needle might need to be set richer (maybe an 1/8 turn more). He said the engine can start to run leaner as it heats up so that might by why the problem that I have only occurs after I’ve driven for about 5 minutes. Unfortunately I haven’t been able to get to the track to try different things yet but I should be on track this Friday. I’ll let everyone know if anything works.

Thanks Walt, I’ll try to find a way to keep a little more fuel in the bowl one way or another.

The low speed jet fix sounds odd, why would it be directional? Maybe you’re barely on a threshold.

I’ve cleaned my carb passages as a first attempt (and the screen filter), if that doesn’t help I’ll try bending the arms. Diaphragm type fuel pump has 4 races since a rebuild, I’m expecting to rebuild each season so that should be OK.

I’m rotax, so my pop-off pressure isn’t controllable, however measuring it would probably teach me something. I can bend the float arms or use heavier floats to increase the amount of fuel in the carb bowl.

My ribs are recovering, I’m hoping to make the next club race July 5th. I’m changing a bunch of stuff during my forced hiatus so it’s going to be difficult to tell what’s working this time around.

Finally got back on the track yesterday. I rotated my airbox so the holes faced outward instead of toward my leg. Made 3 10 lap runs and it never stumbled. So this problem had to have been created by my leg covering the airbox holes.